mostheadphones are made to fit into ears and therefore moulded for left ear canal and right ear canal but also the way music is recorded it is in stereo left output and right output so I guess also marked L and R to make sure you recieve the right signal to the correct ear as the recordings intended you to hear it

It is reasonable to wonder why you need to know right from left when using headphones to listen to music. After all, if you were at a concert there would be sound entering the left ear, and sound entering the right ear as you face the stage, however turning around and facing the other way (easy to do at many rock concerts) reverses which sound is entering which ear and has no effect on the auditory experience. So why worry about switching earphones--the equivalent of turning around.

We should remember that not all recorded materials are stereo music. Some things actually may be intended to "pass by" the listener in a specific progression. Therefore the correct orientation of the earphones would be important.

It is also important to remember personal hygiene. There are bacterias and molds which can be transferred between even an individual's own ears causing some infections. Keeping the correct earphone in the same ear every time reduces this likelihood.